1. Field of the Invention
The present invention relates to a smart battery charging system applied to portable computers, such as notebook computers, and more particularly relates to a smart battery charging system, a charging method therefor and a power supply system for portable computer using the same that executes various charging operations securely corresponding to the type of the smart batteries connected to the notebook computer.
2. Discussion of Prior Art
Recently, portable computers of compact size, such as laptop or notebook computers, have become popular and are in widespread usage in many countries. Typically, these portable computers utilize various battery powers when an external power is not convenient or is not available. At present, lithium ion (Li-ion) and nickel-metal hydride (Ni-MH) batteries have been known to be used in applications for such portable computers. However, since these batteries may burst when over charged, etc., careful attention is necessarily required in designing the concerned charging circuitry. Accordingly, smart batteries, providing their status data, such as charging status, temperature status, etc., to a smart battery charging system for stable and secure charging operations, are in widespread usage as a battery power when an external power is not convenient or is not available.
FIG. 1 shows a port pattern of a general smart battery, comprised of a positive port (+), a resistance sensing port T, a SM bus data port D, a System Management (hereinafter SM) bus clock port C and a negative port (-), that outputs various data, such as over charged alarm, charge terminated alarm, over temperature alarm, fully charged alarm and fully discharged alarm through the SM bus data port T and SM bus clock port C. In particular, the resistance sensing port T is connected to a ground, either through a resistor having a value of 500 .OMEGA. in case of the Li-ion battery, or through a temperature sensor of negative thermal condition NTC type having values of approximately 10 k.OMEGA. to 2.5 k.OMEGA. according to the battery temperature status in case of the Ni-MH battery.
Meanwhile, there are several problems in constructing a smart battery charger for the smart battery described above.
First, in general, there are two types of the smart batteries, Li-ion battery and Ni-MH battery, the former needs a constant voltage charge and the later needs a constant current charge. FIG. 2a is a characteristic curve showing a charging characteristic of the Li-ion smart battery. When constant voltage of 12.30V (12.45V maximum) is applied to the positive port (+) of the Li-ion smart battery during a predetermined time t1, the charging voltage increases gradually and then the Li-ion smart battery is fully charged. FIG. 2b is a characteristic curve showing a charging characteristic of the Ni-MH smart battery. When constant current of 1800 mA maximum is applied to the positive port (+) of the Ni-MH smart battery during a predetermined time t2, the charging voltage is increased gradually to 12.30V, in this manner, the Ni-MH smart battery is fully charged. In particular, when the charging current is supplied to the Ni-MH smart battery continuously after the battery is fully charged at a point of t2, the charging voltage starts to be decreased on the contrary. Therefore, it needs to shut off the charging current when the smart battery is fully charged.
Meanwhile, it is expected that the user of a notebook computer may not be aware of the different characteristics of the two smart batteries applied to the notebook computer and make the Li-ion smart battery supplied with a higher charging voltage than a regular charging voltage or make the Ni-MH smart battery charged by a higher charging current than a regular charging current. This may cause explosion of the corresponding smart battery.
Therefore, when applying the above described smart batteries to the notebook computer, it is necessary to execute an appropriate charging operation by identifying the types of the smart batteries being connected, and to arrange circuitry for controlling the charging voltage and the charging current simultaneously against the smart battery.
Second, as described above, while an allowable error limit for the charging voltage of the Li-ion smart battery is within 12.3V+/-1%, if over-voltage, exceeding the allowable error limit, is supplied to the Li-ion smart battery as a charging voltage, the Li-ion smart battery may burst. So, it is indispensable to control the charging voltage for the Li-ion smart battery precisely.
Third, the smart battery charging circuitry reads out the smart battery status through the SM bus and executes charging control operations according to the result. Meanwhile, in case that the SM bus doesn't operate due to a short, etc., and the smart battery is over charged accordingly, the smart battery may burst. So, it is necessary to provide a safety device for preventing the smart battery from being over charged securely.
And, in case that the smart battery described above is applied to the notebook computer, when the user turns off the notebook computer, leaving the notebook computer being connected to an external power source, and if the charging control circuitry of the smart battery is set to a non-operational status, the charging operations will be continuously executed, regardless of the charge status of the smart battery. This may also cause explosion of the smart battery. So it is necessary to keep the charging control circuitry of the smart battery in an operational status constantly, regardless of the power on-off status of the notebook computer.